Polyisocyanates have had wide study as adhesives and binders in forming composite materials. For example, polyisocyanates have been used to manufacture wood products such as hardboard and particle board. Each such product generally has a defined scope of industrial utility, rendering it useful for certain applications but not others.
Representative prior disclosures of polyisocyanate-containing composite materials include the following:
U.S. Pat. No. 3,666,593 to Lee discloses overlaying a rigid substrate, such as wood, metal, plastic, or asbestos board, with one or more layers of polyisocyanate-impregnated paper. The polyisocyanate-impregnated paper is cured at 100 to 350 psi at a temperature from 280.degree. to 340.degree. F.,
U.S. Pat. No. 4,359,507 to Gaul et al. discloses particle boards made using lignified cellulose and an adhesive binder polyisocyanate and a viscosity-reducing diluent such as ethylene carbonate, propylene carbonate, or lignin. The binder has 20 to 95% w/w polyisocyanate, preferably 50 to 75% w/w, relative to the mass of the mixture of polyisocyanate and diluent.
U.S. Pat. Nos. 4,486,557, 4,344,798, 4,361,662, and 4,414,361 to Gaul et al. disclose various adhesive binder composition for lignified cellulose comprising polyisocyanate and one or more diluents that serve principally to "extend" the polyisocyanate resin.
Gaul et al., "Novel Isocyanate Binder Systems for Composite Wood Panels," Proceedings of the S.P.I. International Technical/Marketing Conference, San Diego, Cal., Nov. 2-4, 1983, pp. 389-407, teaches that dilution of polyisocyanates such as poly(diphenylmethane diisocyanate) (termed "PMDI") with progressively increased amounts of a miscible organic diluent yields particle boards exhibiting progressively decreased internal bonding strength. PMDI dilution also results in decreased modulus of rupture and modulus of elasticity.
European patent application No. 0,304,273 to Younes discloses composite materials in which a lamina of metal, cellulose, plastic, glass, or ceramic is compression-molded with a rigid polyisocyanurate lamina. The amount of cyclic alkylene carbonate is about 2 to 50% w/w, preferably 20 to 35% w/w, relative to the mass of the reaction mixture.
U.S. Pat. No. 5,008,359 to Hunter discloses a polyurea-cellulosic composite material made using an essentially uncatalyzed polyisocyanate resin.
It is also known to make laminated composites having a phenolic resin impregnated paper substrate with an overlay of a melamine containing material. These materials have been widely applied, for example as countertop coverings in residential and commercial establishments. One such prior art covering is sold under the brand name Wilsonart.RTM., while another is sold under the brand name Formica.RTM.. The precise construction of these materials is unknown, but they are understood to be formed of a melamine-plastic overlay on a phenolic impregnated paper substrate. These materials must be able to resist denting or deformation from objects dropped thereon. Although the Wilsonart.RTM. covering material has been well received, improvements in the durability, cost reduction and environmental impact of such materials would be desirable.
Therefore, a need exists for improved materials of this type.